The present invention relates to a magnetostrictive torque sensor and a magnetostrictive torque measuring apparatus, as well as to a condition-monitoring apparatus for a cutting tool.
A magnetostrictive stress (torque) sensor makes use of a phenomenon in which magnetic permeability changes when a mechanical strain is imparted to a ferromagnetic material, the phenomenon being one of the phenomena of magnetic strain. The magnetostrictive torque sensor is arranged such that a rotary shaft for the detection of torque is formed by a ferromagnetic material such as iron, and two U-shaped coils (each arranged by winding a coil around open-side opposite legs of a U-shaped core), which form a bridge in such a manner as to oppose side surfaces of the shaft member, are disposed in a crossing manner.
Further, one core is disposed parallel to the axial direction of the rotary shaft, and the other core is disposed in a direction perpendicular to the axial direction. In other words, the legs of the two cores are positioned at vertices of a square, but the sides of the square are positioned in such a manner as to be inclined 45 degrees with respect to the axial direction of the rotary shaft.
If torque is applied to the rotary shaft in this state, the rotary shaft is twisted and a tensile stress and a compressive stress occur in a direction offset .+-.45 degrees with respect to the center line of the shaft. As a result, the magnetic permeability at the rotary shaft changes. With respect to the change of permeability, an a.c. current is allowed to flow across one of the two U-shaped coils to excite it, and a signal is fetched from the other coil, so as to detect the change in permeability and determine the torque on the basis of the result of detection thereof.
With the above-described conventional sensor, however, the following problems are encountered. Namely, the magnetic pole portions have a magnetic path which include an open end of the exciting core, the interior of the shaft subject to measurement, an open end of the detecting core, the interior of the detecting core, the other open end of the detecting core, the interior of the shaft subject to measurement, and the other open end of the exciting core. Since the exciting core and the detecting core are arranged in a crossing state, there are two systems of the aforementioned magnetic path, and the torque is detected as the magnetic permeability in the respective magnetic paths changes differentially with respect to the direction of the torque applied to the shaft subject to measurement.
However, the magnetic path passing through the interior of the shaft subject to measurement between the open end of the exciting core and the open end of the detecting core is present at four different positions. Accordingly, the permeability at sites which are adjacent to each other but vary in position is inevitably measured. Since the permeability in the shaft to be measured is nonuniform, large zero-point fluctuations occur as the consequence of the rotation of the shaft.
Moreover, in accordance with this system, it is possible to detect a torque in a certain direction with good sensitivity, but the detection sensitivity with respect to a torque by which the shaft is twisted in the opposite direction is low. Accordingly, in order to increase both detection sensitivities of torque in the two directions, it is necessary to provide two sensors of the above-described arrangement, and dispose them at axially different positions. Hence, the aforementioned problem (the zero-point fluctuations and the like as the consequence of the rotation of the shaft) become more noticeable, and the apparatus becomes large in size.
In addition, the magnetic pole which is formed by each coil becomes the open end face of each core, so that there occurs the problem that the area of magnetic pole is small and the sensitivity becomes low.